1. Field of the Invention
The apparatus of the present invention relates generally to treatment of articles in a vacuum environment, and in particular to a system for stabilizing the temperature of such articles.
2. Description of Related Art
Numerous semiconductor manufacturing processes which produce pattern masks on transparent substrates require temperature stabilization of the substrate and protective carrier prior to pattern writing to prevent pattern distortion resulting from thermal expansion or contraction during the writing process. Temperature stabilization requirements are typically .+-.0.05 degrees C. relative to the writing chamber temperature.
The traditional temperature stabilization method utilizes long soak periods (&gt;8 hours) in a temperature controlled "environmental" chamber at atmospheric pressure. This method removes initial temperature differences in the substrates and references the substrate temperature to the "environmental" chamber. In some configurations, the substrates and carriers are loaded by "hand" from a separate "environmental" chamber to a vacuum load lock where the atmosphere is evacuated. This "hand" loading can cause a significant temperature deviation of 0.1.degree.-1.0.degree. C. in the substrate from heat transferred from the operator's hand, typically 10.degree.-15.degree. C. above room ambient temperature. However, prior to writing the pattern, the gas environment must be evacuated from the load lock (typically to 1E-7 Torr) which cools the substrate due to gas expansion cooling. This evacuation typically causes a 6".times.6".times.0.090" thick glass plate to lose 0.6.degree.-0.9 degrees C. Since production requirements typically require 2 or more substrates per hour, insufficient time is available for a second temperature stabilization soak process.
Also, substrate preheating attempts to offset the evacuation cooling effect are not totally effective since contact to the substrate image area (usually 90% or more of substrate) is prohibited which makes substrate temperature monitoring inaccurate and prevents surface contact heating methods. Also, gas convection heating exposes the substrate to particulate contamination from the gas supply or particulates in the chamber stirred by air currents.